GB News has been fined £100,000 for breaking impartiality rules over a programme featuring Rishi Sunak, Ofcom has said.
It comes after the media watchdog announced in May that the show called People's Forum: The Prime Minister had breached broadcasting guidelines.
The programme featured then prime minister Mr Sunak answering questions from a studio audience and a presenter.
GB News chief executive Angelos Frangopoulos said the fine was a "direct attack on free speech and journalism in the United Kingdom".
"We believe these sanctions are unnecessary, unfair and unlawful," he added.
The hour-long show, which aired on 12 February, prompted 547 complaints to Ofcom.
The regulator found earlier this year that while featuring Mr Sunak was fine in principle, "due weight" should have been given to an "appropriately wide range of significant views" other than the Conservatives.
Ofcom said Mr Sunak "had a mostly uncontested platform to promote the policies and performance of his government in a period preceding a UK general election," which it recorded as a breach of impartiality rules.
The watchdog said "given the seriousness and repeated nature of this breach," it had imposed a £100,000 financial penalty.
Read more from Sky News:
CCTV shows car smash into PM's Chequers estate
See if you are better or worse off using our budget calculator
GB News was also directed to "broadcast a statement of our findings against it, on a date and in a form determined by us".
The TV channel is challenging the breach decision by judicial review and Ofcom will not enforce the sanction decision until those proceedings are concluded.
Follow Sky News on WhatsApp
Keep up with all the latest news from the UK and around the world by following Sky News
Mr Frangopoulos insisted the show featuring Mr Sunak "was an important piece of public interest programming", and that "appropriate steps" were taken to ensure due impartiality.
He added: "It was designed to allow members of the public to put their own questions directly to leading politicians.
"GB News chooses to be regulated and we understand our obligations under the Code.
"But, equally, Ofcom is obliged by law to uphold freedom of expression and apply its rules fairly and lawfully."
Ofcom previously found GB News violated due impartiality rules in March over five programmes that featured Tory MPs as presenters.